[Haskell-cafe] space leak when repeatedly calling Control.Monad.State.Strict.modify

Joey Hess joey at kitenet.net
Mon Jan 30 03:36:14 CET 2012

Claude Heiland-Allen wrote:
> Control.Monad.State.Strict is strict in the actions, but the state
> itself is still lazy, so you end up building a huge thunk in the
> state containing all the updates that ever took place to the initial
> state.
> Using this should fix it:
> modify' :: MonadState s m => (s -> s) -> m ()
> modify' f = do
>   x <- get
>   put $! f x  -- force the new state when storing it


So, why does Control.Monad.State.Strict.modify not do that?

And, I still don't quite understand why this only happened
when the updated value is obtained using IO.

see shy jo
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